The present invention relates to the field of fuses, more particularly to microfuses.
Microfuses are physically small fuses typically used to protect electronic components used in transistorized circuitry, such as televisions, radios, computers, and other devices requiring physically small circuit interruption devices. A typical microfuse may be about 1/2 of an inch long and about 1/10 of an inch wide.
One prior art microfuse that is suitable for high speed automated assembly employs a ceramic substrate having metallized weld pads on the opposed ends thereof, having wire leads attached thereto, and a fusing link in the form of a wire ultrasonically bonded to the metallized weld pads. The substrate, with pads and wire thereon, may be coated in an arc quenching media, and then coated in a protective coating such as plastic.
The microfuse employing an ultrasonically bonded fusing wire has a limited range of ratings. The minimum diameter of the automatically bonded wire is too large to allow the fuse designer to achieve a fractional amperage fuse. Further, small diameter fuse wires are fragile, and as a result, the manufacture of microfuses employing such wires requires special handling to reduce the incidence of fuse wire breakage.
In response to the breakage and handling problems associated with fuse wires used in microfuses, thick film fusing links have been proposed to replace the wire fusing link in the microfuse. The thick film element is deposited directly on the substrate typically by screen printing a conductive ink thereon. A mask is used to create a pattern having opposed welding pads for receiving fuse lead wires and a narrowed portion therebetween forming a fusing link. To change the ampere rating of the fuse, the minimum cross-sectional area of the narrowed portion (or weak spot) of the fuse is varied. For a given material for the fusing link, the narrower the cross-section, the lower the current required to cause the fuse to open. The physical properties of the thick film ink limit the minimum width of the weak spot to 2 to 8 times the typical thickness of 500 micro-inches. This minimum cross-sectional area of the thick film weak spot is too large to manufacture fuses having a rated capacity below approximately 1 amp for fuse link materials of silver. Fuse link materials with higher resistivity can be used, but they result in microfuses that have higher resistance, voltage drops and body temperatures and less interrupting ability.
A more effective way to reduce the amperage rating of the fuse, is to make the fusing link and weld pads of different thicknesses. This is best achieved by printing the fuse link with a thin film ink or by the deposition of a thin film using vapor deposition, sputtering, or chemical vapor deposition techniques. However, it has been found that where the thickness of the fusing link falls below approximately 100 micro-inches, the surface roughness of the substrate causes large variations of the thickness of the material forming the fusing link on the substrate, which leads to erratic fuse resistance and performance. Such erratic performance includes fuses having characteristics out of specification such as opening times, voltage drops and open fuses prior to use.
A typical ceramic substrate has an average surface roughness of approximately 10 to 40 micro-inches. A glass-coated ceramic substrate, however, has an average surface roughness of 0.06 micro-inches. Thus, a thin film metalization with a thickness of 6 micro-inches provides a continuous layer with less than 1% cross sectional area variation. The glass layer is 2,300 micro-inches thick.
If the entire ceramic chip is coated with glass, however, then a second problem is encountered. To achieve high speed automated assembly of the microfuse, the external leads are resistance welded to the metalized pads at the ends of the ceramic chip. The strength of this welded joint is not acceptable if there is a glass layer between the metalization and the ceramic substrate. The thermal shock of the resistance welding operation produces microcracks in the glass layer.
The inability to manufacture microfuses (with high speed automated equipment) having amperage ratings of less than 1 amp has denied the electronics industry a low cost fractional amperage microfuse.
The present invention overcomes these deficiencies of the prior art and permits the high speed automated manufacturing of microfuses in the 1/32 to 1 amperage range.